A spark plug for an internal combustion engine, such as an automotive engine, generally includes a center electrode, a substantially cylindrical insulator disposed around the center electrode, a substantially cylindrical metal shell disposed around the insulator and a ground electrode attached as desired to a front end of the metal shell. One such type of spark plug is disclosed in Patent Document 1. Patent Document 1 specifically recites therein “a spark plug comprising: a shaft-shaped center electrode; a metal shell disposed on an outer circumferential surface of the center electrode; a ground electrode having one end coupled to the metal shell and the other end facing the center electrode; and a spark plug insulator according to any one of claims 1 to 4 arranged to provide insulation between the center electrode and the metal shell” (see claim 5).
The structure of the internal combustion engine to which the spark plug is mounted has recently been becoming more complicated. There is thus a demand to reduce the size such as diameter of the spark plug in order to secure the design flexibility of the internal combustion engine. It is effective to decrease the wall thickness of the insulator for the purpose of downsizing the spark plug. However, the spark plug cannot maintain its withstand voltage characteristics when the wall thickness of the insulator becomes decreased. On the other hand, it is effective to keep the outer diameter of the center electrode and the wall thickness of the insulator similar to those of conventional spark plugs for the purpose of downsizing the spark plug while securing the durability and withstand voltage characteristics of the spark plug. In such a case, the clearance between the insulator and the metal shell in the ignition part (sometimes referred to as “gas volume”) becomes narrowed. When the clearance between the insulator and the metal shell becomes narrower than required, the spark plug fails to generate a proper spark discharge in a spark discharge gap between the center electrode and the ground electrode but causes a so-called lateral spark between an outer circumferential of the insulator and an inner circumferential surface of the metal shell. This results in a deterioration of the ignitability of the spark plug.
By the way, high-compression internal combustion engines and internal combustion engines with superchargers have recently been developed for the purpose of improvements in engine performance and efficiency. In each of these internal combustion engines, the ignition part of the spark plug is subjected to extreme conditions due to increases in the ambient temperature and discharge voltage of the spark plug. When the spark plug, particularly the downsized spark plug, is subjected to such extreme conditions, the ignitability of the spark plug could significantly deteriorate during high-load operation of the internal combustion engine. The reasons for such ignitability deterioration are assumed as follows. As the front end of the insulator is heated to a high temperature during high-load operation of the internal combustion engine, there occurs softening of a glass component in the insulator so that electrons of the glass component move actively to generate a flow of electric current in the glass phase of the insulator. The generated electrical current is fed to the outer circumferential surface of the insulator and discharged to the outside air. Namely, the electric current for the spark discharge is leaked out of the insulator. The spark plug cannot receive a required amount of energy for the spark discharge and thus cannot generate a proper spark discharge within the spark discharge gap between the center electrode and the ground electrode. Hereinafter, the electric current flowing through the glass phase of the insulator and leaked out of the insulator is referred to as “leakage current” in the present invention.
The ignitability deterioration of the spark plug directly leads to a deterioration of emission performance. It is particularly important to solve such an ignitability deterioration problem in these days where environmental measures are required.